1. Field of the Invention
This invention relates to an improved hydrogenation catalyst, and a method for using the catalyst in a hydrogenation process. More specifically but not by way of limitation the invention relates to the production of 1,4-butanediol and gamma-butyrolactone and mixtures thereof from a hydrogenatable precursor such as maleic acid, succinic acid, their anhydrides, esters, gamma-butyrolactone and mixtures thereof in an acidic aqueous medium in the presence of hydrogen and a catalyst consisting essentially of reduced or at least partially reduced ruthenium and tin on an aqueous acid insoluble refractory oxide support.
2. Description of Related Art
Various methods and reaction systems have been proposed in the past for manufacturing tetrahydrofuran (THF), 1,4-butanediol (BDO) and gamma-butyrolactone (GBL) by catalytic hydrogenation of maleic acid, succinic acid, their anhydrides, esters, and/or related hydrogenatable precursors. Also a variety of hydrogenation catalysts have been historically proposed for this purpose including Group VIII noble metals, combinations of them and also their combinations with rhenium, tin and germanium deposited on various inert supports. These methods and the catalysts described heretofore produce different amounts of a number of hydrogenated products. For example, U.S. Pat. No. 4,609,636 describes the use of a catalyst composite comprising palladium and rhenium on a carbon support for making THF, BDO or mixtures thereof from a variety of hydrogenatable precursors. Also Japanese patent application publications (Kokai) 6-157490, and 6-179667 disclose methods for preparing THF by catalytic hydrogenation of maleic anhydride or maleic acid in the presence of an acidic substance using as a catalyst a rhenium compound and a Group VIII metal. Comparative examples illustrate the use of a catalyst comprising ruthenium and rhenium supported on a carbon support without the acidic substance being present.
Methods are known in the art for the selective production of THF by the catalytic reduction of hydrogenatable precursors. For example, U.S. Pat. No. 4,609,636 teaches that the relative ratio of THF to BDO can be increased by increasing one or more variables selected from operating temperature, contact time, and hydrogen space-time. It is also known from numerous references such as U.S. Pat. No. 3,726,905, that dehydration of BDO to give THF is catalyzed by acid and that increasing the acid concentration results in an increase of the relative ratio of THF to BDO. U.S. Pat. No. 5,478,952 relates to an improved catalyst and method for the production of THF, GBL and BDO by hydrogenation in aqueous solution; the catalyst consisting essentially of highly dispersed, reduced ruthenium and rhenium on a carbon support. THF and GBL are the principal products from this process, THF usually predominating and only minor amounts of BDO and GBL being produced. Japanese patent application publication (Kokai) 5-246915 teaches the use of any Group VIII noble metal, including palladium and ruthenium in combination with either tin, rhenium or germanium as a catalyst for aqueous phase hydrogenation of carboxylic acids or esters. The reference distinguishes the claimed subject matter from previous aqueous phase hydrogenation catalysts incorporating these metals by virtue of specifically claiming the use, as carrier, of porous carbon having a BET surface area of at least 2000 m.sup.2 /g. The main products which result from this process are THF and GBL. Japanese patent application publication (Kokai) 6-239778 describes the catalytic hydrogenation of maleic acid using supported ruthenium and tin bimetallics. In the examples silica is the support. Titania and zirconia are disclosed also as supports. It is a feature of this catalyst that performance is improved by the addition of alkali metal compounds or nitrogen containing bases. Yields of 30 to 50% BDO are obtained in autoclave reactions with GBL constituting the other major product.
Japanese patent application publications (Kokai) 6-116182 and 7-165644 also relate to the use of ruthenium and tin bimetallics for the catalytic hydrogenation of maleic acid to produce THF, GBL and BDO. In the first reference the preferred support is silica modified with titania or alumina and the highest yield of BDO shown in the examples is 3.5%. Comparative examples are described in which non-modified silica and titania are the supports. In both cases the yield of BDO is below 1%. In the latter reference the addition of platinum and rhodium is also necessary and silica is the only support used in the examples, with titania and silica being disclosed as possible supports. The highest yield of BDO shown in the examples is 15%.
An important area subject to improvement is selectivity with respect to the products of a catalytic hydrogenation process. Selectivity is defined herein to refer to a measure of the percentage of the principal products of a process which in the case of this invention are BDO and GBL. The processes described heretofore do not teach how BDO or GBL or their mixtures can be selectively produced while producing only small amounts of THF, alkanols, carboxylic acids and alkanes as byproducts. The greatly improved selectivity obtained with the process of this invention is of important commercial significance since it allows for the more economical production of BDO and GBL, two commodities with a variety of uses.
Both BDO and GBL can be readily converted to THF which is a widely used solvent in many processes. THF is an excellent solvent for high polymers such as polyvinyl chloride and as a monomer it is used in the production of polyether polyols.